An electronic circuit that is known in the art comprises a constant-voltage generation circuit that outputs a constant voltage and a crystal oscillation circuit that is driven by this constant voltage. This kind of electronic circuit is widely used in applications such as timepieces, telephones, and computer terminals.
Recent trends in the miniaturization of electronic equipment have made it particularly common to fabricate such electronic circuitry as compact, low-power ICs.
However, electronic circuitry formed as an IC has a problem in that a constant voltage that is output from the constant-voltage generation circuit varies with the effects of temperature.
This is particularly important with a crystal oscillation circuit that is driven by a constant voltage output by a constant-voltage generation circuit because, if that constant voltage should change, the oscillation frequency of the crystal oscillation circuit will also change. This causes a problem in an electronic circuit that generates a reference clock signal for operation based on the oscillation frequency of this crystal oscillation circuit, in that accurate time-keeping is not possible. If a wristwatch is taken by way of example, the environment in which such a wristwatch is used can range from low temperatures to high temperatures. If prior-art electronic circuitry is used in such a wristwatch, therefore, variations in the constant voltage that is output from the constant-voltage generation circuit can cause errors in the time displayed thereby.
It is necessary to set the absolute value of the constant voltage that is output from the constant-voltage generation circuit to be always equal to or greater than the absolute value of the oscillation-stopped voltage of the crystal oscillation circuit. If this voltage falls below the oscillation-stopped voltage, the crystal oscillation circuit will no longer be able to operate.
It is known that the power consumption of the crystal oscillation circuit is proportional to the square of the constant voltage supplied from the constant-voltage generation circuit. To reduce the power consumption of the electronic circuitry, therefore, it is necessary to set the value of the constant voltage that is output from the constant-voltage generation circuit to be as small as possible, within a range that satisfies the condition that it is equal to or greater than the oscillation-stopped voltage of that crystal oscillation circuit.
If such electronic circuitry is formed as a semiconductor integrated circuit, the effects of factors such as errors in impurity implantation will cause subtle changes in the value of the constant voltage output from the constant-voltage generation circuit and the value of the oscillation-stopped voltage of the crystal oscillation circuit.
Since it is not possible to finely adjust the value of the constant voltage that is output from the constant-voltage generation circuit in prior-art electronic circuitry, it is necessary to set the value of this constant voltage to have a sufficiently large margin over the expected value of the oscillation-stopped voltage, from consideration of the risk of a large variation therein. This means that the crystal oscillation circuit is driven by a voltage that is larger than necessary, raising a problem in that it is difficult to reduce the power consumption of the electronic circuitry from this aspect too.
The present invention is devised in the light of the above problems and has as a first objective thereof the provision of an electronic circuit, semiconductor device, electronic equipment, and timepiece wherein the value of the constant voltage that is output from the constant-voltage generation circuit is not affected greatly by changes in temperature.
Another objective of this invention is to provide an electronic circuit, semiconductor device, electronic equipment, and timepiece wherein the value of the constant voltage that is output from the constant-voltage generation circuit can be adjusted finely.